1. Field of the Invention
This invention relates to a method for making magnetic cores for use in electrical apparatus, and more particularly, it pertains to microlaminations having coatings of hydrated magnesium silicate.
2. Description of the Prior Art
All particulate cores require insulation of particles if eddy current losses are to be low. Fine powder cores are impractical to insulate unless insulation also bonds cores together.
Iron, low carbon steel, or silicon steel particles which are made into magnetic cores by powder metallurgy techniques require an electrically insulative coating on them to minimize eddy current losses in alternating current applications. Such particles, other than those of the socalled "microlaminations," are obtained either by slitting and chopping thin gauge strip, reclaiming the scrap from chip removal operations, such as sawing or machining, or by various other comminutive processes. Generally, the particles are cleaned if necessary, decarburized, annealed, coated with an insulative material, typically magnesium methylate (Mg(OCH.sub.3).sub.2) in eight weight percent solution in methanol, and then pressed into final shape such as a magnetic core.
The insulative coating has several severe requirements. First, it must be extremely thin so that the compacted particles will have a very high packing factor, i.e. a high ratio (&gt;0.9) of core density to theoretical density. The higher the packing factor can be made, the greater will be the magnetic permeability. Second, the coating must cover the particle surfaces thoroughly, particularly at edges, corners and asperities so that each particle is insulated from its neighboring particles in the pressed core. The better the microlaminations are covered, the lower will be the core loss. Third, the coating must withstand elevated temperature since in many applications it is desirable to anneal the compacted core in order to lower the coercive force and ore loss and raise permeability. Fourth, the coating must withstand extensive deformation and abrasion during the pressing operation yet still provide adequate interparticle insulation after pressing. Fifth, the coating must be cheap, readily available and easily applied to the microlaminations.